Agriculture Reference
In-Depth Information
rays and, to a lesser extent, from the bud shield, and rupture through the
necrotic plate. Callus originating almost entirely from rootstock tissue, mainly
from the exposed surface of the xylem cylinder, surrounds the bud shield and
holds it in place and, over a period of two or three weeks, fills all internal
air pockets. A continuous cambium is then established, isolated tracheary
elements appear and the callus lignifies. This lignification is completed by
about
). In
chip budding, in which the bud is removed from the scion as part of a wedge
of tissue which is then seated in a corresponding notch on the rootstock stem,
the juxtaposition of the xylem and cambial tissues of stock and scion results in
the formation of a much more rapid and complete union (Figure
weeks after budding (Mosse and Labern,
; Wagner,
.
).
Incompatibility
When two different plants can be grafted together to produce a long-lived,
functional graft union and to develop successfully into one composite plant,
they are said to be compatible. When this is not the case they are said to
be incompatible. In general the more closely related the plants the higher is
the chance of compatibility although permanent unions between one genus
and another may occur. Pear (
Pyrus communis
) will form a lasting union with
hawthorn (
Crataegus oxyacantha
), medlar (
Mespilus germanica
) and quince (
Cydonia
oblonga
). Although apple and pear are usually incompatible the pear culti-
var 'Fertility' has exceptionally high compatibility with apple and 'M.
' ap-
ple rootstock clone with pear (Garner,
). However, some economically-
important pear cultivars are incompatible with otherwise desirable quince
rootstocks and some apple cultivars with otherwise desirable apple rootstocks,
whileincompatibilitycanbeinducedbetweenotherwisecompatiblerootstocks
and scions by viruses and phytoplasmas.
Incompatibility has the following symptoms:
Initial failure to form graft or bud unions.
Poor growth of the scion often followed by premature death in the nursery:
premature yellowing of foliage.
Breaking off of trees at the graft union, especially when they have been
growing for many years and the break is clean and smooth.
The incompatibilities of the greatest commercial importance in apples and
pears are:
. Pear-quince incompatibility. When some pear cultivars are grafted on
quince rootstocks a cyanogenic glycoside, prunasin, which is normally found
in quince but not in pear is translocated into the pear phloem. The pear tissues
